Somatic embryogenesis in Arabidopsis thaliana is facilitated by mutations in genes repressing meristematic cell divisions

Embryogenesis in plants can commence from cells other than the fertilized egg cell. Embryogenesis initiated from somatic cells in vitro is an attractive system for studying early embryonic stages when they are accessible to experimental manipulation. Somatic embryogenesis in Arabidopsis offers the additional advantage that many zygotic embryo mutants fan be studied under in vitro conditions. Two systems are available. The first employs immature zygotic embryos as starting material, yielding continuously growing embryogenic cultures in liquid medium. This is possible in at least 11 ecotypes. A second, more efficient and reproducible system, employing the primordia timing mutant (pt allelic to hPt, cop2 and amp1), was established. A significant advantage of the pt mutant is that intact seeds, germinated in 2,4-dichlorophenoxyacetic acid (2,4-D) containing liquid medium, give rise to stable embryonic cell cultures, circumventing tedious hand dissection of immature zygotic embryos. pt zygotic embryos are first distinguishable from wild type at early heart stage by a broader embryonic shoot apical meristem (SAM). In culture, embryogenic clusters originate from the enlarged SAMs. Pt somatic embryos had all characteristic embryo pattern elements seen in zygotic embryos, but with higher and more variable numbers of cells. Embryogenic cell cultures were also established from seedling, of other mutants with enlarged SAMs, such as clavata (clv). Pt clv double mutants showed additive effects on SAM size and an even higher frequency of seedlings producing embryogenic cell lines, pt clv double mutant plants had very short fasciated inflorescence stems and additive effects on the number of rosette leaves. This suggests that the PT and CLV genes act in independent pathways that control SAM size. An increased population of noncommitted SAM cells may be responsible for facilitated establishment of somatic embryogenesis in Arabidopsis.